1. Field of the Invention
This invention relates to a projection lens, and in particular to a projection lens for use in a video projector for projecting a CRT image and obtaining a large picture plane.
2. Related Background Art
In recent years, so-called video projectors are gradually being popularized as a method for obtaining television reproduced images of large picture plane, and in securing the quality of the reproduced images, the performance of a projection lens bears an important role.
In order to obtain bright projected images, a bright projection lens of a great aperture is required and at the same time, in order to shorten the distance from the CRT image plane to the screen and make the depth of the cabinet as a projection apparatus compact, it is required to make the angle of view of the projection lens wide. Generally, in a video projector, three projection lenses corresponding to CRTs of three colors, B(blue), G(green) and R(red), and various projection lenses using aspherical plastic lenses have been devised to achieve the above-described high-degree specification while making the lenses compact and light in weight and reducing the cost of manufacture.
The projection lens disclosed, for example, in U.S. Pat. No. 4,548,480 comprises three lenses as shown in FIG. 1 of the accompanying drawings. That is, this projection lens comprises, in succession from the screen side (the left side as viewed in FIG. 1), a first lens component G1 having a positive refractive power, a second lens component G2 of biconvex shape having a positive refractive power, and a third lens component G3 having a negative refractive power and having its surface of sharper curvature facing the screen side. Aspherical surfaces are used in the first lens component G1 and the third lens component G3 to thereby correct the imaging performance well, but with regard to distortion, as shown in FIG. 2 of the accompanying drawings, distortion is deflected at the medium angle of view in the positive direction and deflected at the outermost marginal portion in the negative direction, and bending of so-called high-order aberrations is created. Such a distortion characteristic is considered to be attributable chiefly to the structure of the third lens component G3. That is, the third lens component is disposed at a position nearest the CRT and has the function of correcting chiefly curvature of image field as a so-called field flattener, but a projection lens having a great aperture and a wide angle of view is of a shape in which the curvature of the surface thereof which is adjacent to the screen becomes gentler away from the center of the optic axis to prevent deterioration of off-axis coma. As a result, the action of the negative lens for deflecting distortion in the positive direction becomes weaker in the marginal portion thereof, and this presents itself as the bending of distortion.
Such bending of distortion presents itself as a more remarkable tendency in an arrangement wherein the spacing D.sub.2 between the second lens component and the third lens component is short, because the refractive power of each lens component must unavoidably be made strong.
Specifically, this corresponds to an arrangement in which the spacing D.sub.2 between the second lens component and the third lens component must be shortened to secure a back focal length, such as (1) a case where silicon gel is enclosed for the purposes of preventing any reduction in contrast by the reflection on the third lens component and the surface of the face plate of the CRT and improving the CRT characteristic by liquid cooling, (2) a case where a mirror inclined at approximately 45.degree. is installed between the first lens component and the second lens component to achieve a compact arrangement, or (3) a case where enlargement of high magnification in which the enlargement magnification exceeds ten times.
Particularly, where high-order bending is present in distortion, bobbin type distortion is combined with barrel type distortion, and this is not preferable.
Moreover, the following problem arises in a projection lens for a video projector. In a popular video projector, as shown in FIG. 3, of the accompanying drawings, the images by G-CRT, B-CRT and R-CRT corresponding to three colors, G(green), B(blue) and R(Red) are compositely projected onto a screen S by respective projection lenses L.sub.B, L.sub.G and L.sub.R, whereby an orthochromatic image is obtained. At this time, from the limitations in the arrangement of the projection lenses, B-CRT and R-CRT are projected from directions each inclined by .theta. with respect to the middle G-CRT. To obtain an image clear-out up to the marginal portion of the picture plane, as is well known, the projection lenses L.sub.B and L.sub.R for B-CRT and R-CRT and the CRTs B-CRT and R-CRT may be inclined with respect to the lens L.sub.G and G-CRT, in accordance with the principle of swing and tilting. In this case, the images provided by the inclined projection lenses L.sub.B and L.sub.R change from a rectangular shape to a trapezoidal shape, as shown in FIG. 4A of the accompanying drawings. So, by partly changing and correcting the scanning magnifications of B-CRT and R-CRT, the shapes of the projected images by B-CRT, G-CRT and R-CRT are accurately superposed one upon another on the screen and an image of natural color tones.
However, where at this time, remarkable high-order bending is present in the distortion of the projection lenses as shown in FIG. 2, the image indicated by solid line in FIG. 4A becomes the image indicated by solid line in FIG. 4B of the accompanying drawings, and the correction on the CRTs becomes difficult. As a result, color misregistration has occurred in the marginal portion of the picture plane and has presented itself as a reduction in quality of image.